Hologram recording sheet, holographic optical element using said sheet, and its production process

ABSTRACT

The hologram recording sheet according to the invention is made up of a base film and hologram sensitive materials sensitive to different wavelength regions formed therein in a desired pattern, or a film and at least two hologram recording sensitive materials sensitive to different wavelength regions laminated on the film with a transparent plastic spacer layer located therebetween, thereby enabling the required diffraction light wavelengths to be recorded on the required sites without producing unnecessary interference fringes. At least two hologram recording sensitive materials sensitive to different wavelength regions are formed on different sites on a film in dotted or striped configuration, the size of which is up to 200 mm or at least twice as large as the thickness of the sensitive material layers, thereby enabling regions diffracting light of different wavelengths to be formed in the form of independent sets of interference fringes.

This is a Continuation of application Ser. No. 08/459,663 filed Jun. 2,1995 now abandoned, which is a divisional of application Ser. No.08/128,143 filed Sep. 29, 1993.

BACKGROUND OF THE INVENTION

The present invention relates to a hologram recording sheet suitable formulticolor displays which is required to selectively diffract light oftwo or more wavelengths, to a holographic optical element such as amulticolor head-up display combiner, a multicolor display hologram, aheat-wave reflecting film that is effectively used on the windows of acar or building, a broad band of holographic filter, and a diffractiongrating which, like the heat-wave reflecting film, diffracts light in awide wavelength range, and to a process for producing such a holographicoptical element.

So far, hologram sensitive materials have been widely used for hologramsfor ornamental, forgery-preventing, optical element or other purposes.Usually, a typical hologram sensitive material is made up of a PETsubstrate on which a hologram sensitive material 2 sensitive to greenand red light is uniformly coated in the form of a single layer, as canbe seen from FIG. 1. This arrangement is applied to a hologram opticalelement required to selectively reflect or diffract light of two or morewavelengths, esp., to a head-up display combiner designed to changecolor from site to site. In a typical combiner, a hologram sensitivematerial capable of diffracting light of plural wavelengths is producedon the entire surface thereof, thereby enabling all colors to bedisplayed.

In the conventional arrangement in which a hologram capable ofdisplaying all colors is provided on the entire surface of the combiner,there is need of diffracting a plurality of light rays having differentwavelengths. As shown in FIG. 2 by way of example, plural sets ofinterference fringes such as interference fringes 4 that diffract greenlight and interference fringes 5 that diffract red light must beproduced in the thickness direction of the hologram. However, not onlydoes this give rise to technical difficulties but also imposes someconsiderable limitation on the hologram recording material to be used.In addition, this is responsible for inferior image quality such asghost images.

One possible approach to solving this problem is the lamination ofhologram sensitive materials sensitive to different wavelength regions.However, high coating technologies such as slide coating, slot coatingand curtain coating are needed for the uniform lamination of manyhologram sensitive materials with their interfaces kept constant, andmuch time is taken to predetermine the required conditions, etc. At thesame time, this approach has a cost problem, because there is need ofusing an exclusive fountain head. Another approach is to use a so-calleddry type of hologram recording material recently put forward by Du Pontor Government Industrial Research Institute, Osaka, which is designed torecord interference fringes by the migration of the monomer containedtherein. However, a grave problem with this is that the unrestrictedmigration of the monomer takes place through the interfaces; in otherwords, the pitch of the interference fringes cannot be obtained, asdesigned, although depending on what type of hologram-recording processis used. Notice that this is quite true when reliance is on sequentialexposure.

On the other hand, attention is now paid to a multicolor displayhologram having excellent effects on ornamentation and preventingforgery and a volume phase (Lippmann) type of hologram excellent inwavelength selectivity and having a profusion of three dimensionaldepth. A hologram combiner is one of such optical elements, andfunctions as a semi-transmitting image-formation element. FIG. 3(a) is aschematic that illustrates how to make a hologram of the image. Lightfrom a laser 10 is split by a half-mirror 11 into two light beams, onetraveling to one point of a lens 12, where it is converted to divergentrays and the other propagating to one point of a lens 13, where it isconverted to divergent rays. These two light rays are incident on bothsides of a volume phase type of hologram recording material 14, forinstance, a photopolymer, so that they can interfere as a Lippmannhologram. This is a hologram combiner.

As shown in FIG. 3(b), such a hologram combiner 16 diffracts lightleaving a display object 15 located in the vicinity of one divergentpoint for image-taking in the reflection direction, so that thediffracted image can emerge as if they came from a display object 15'located in the vicinity of the other divergent point for image-taking.The image-formation magnification and the image position are determinedby the relative distances L and L between the divergent point forimage-taking and the recording material. Hence, this hologram combinerfunctions to diffract only the light of the wavelength for recording ora wavelength having a specific relation thereto and transmit light ofother wavelength, so that the image superposition or synthesis can beachieved. The optical elements represented by such a hologram combinerinclude a head-up display combiner.

Optical systems for recording and reconstructing various color displayholograms will now be explained with reference to FIGS. 4 to 6.

FIG. 4 is a schematic of an optical system for recording andreconstructing a laser light reconstructing hologram. As shown in FIG.4(a), a sensitive material 21 is trebly exposed to light at sequentiallyvarying wavelengths R→G→B, so that light from an object 20 and referencelight can interfere on a recording material 21 to record interferencefringes. For reconstruction, the hologram 21 is illuminated by mixed(R+G+B) light from the same direction as the reference light, as shownin FIG. 4(b), so that the color hologram image of the object 20 can beobserved at the original position.

FIG. 5 is a schematic representing an optical system for recording andreconstructing a rainbow hologram. At the first stage, light from anobject 20 and reference light interfere on three sensitive materials 21at sequentially varying recording wavelengths R→G→B, so that three firstholograms for R, G and B can be produced, as shown in FIG. 5(a). At thesecond stage, the first holograms are illuminated by light on the wavefronts conjugate to the reference waves, so that a real image of theobject is reproduced at the original object position. Then, while asecond sensitive material 23 is located at the position where the realimage is reproduced and a slit is placed just in front of the firstholograms, thereby keeping transverse parallax intact and eliminatinglongitudinal parallax, treble exposure is carried out at varyingrecording wavelengths R→G→B and with varying three first holograms forR, G and B, so that a second hologram is produced by interference withthe reference light, as shown in FIG. 5(b). Upon reconstruction by whitelight on the wave fronts conjugate to the reference waves, an imagehaving a profusion of three dimensional depth is produced in thelengthwise direction of the slit. If the observer's viewing position ismoved vertically to this, the reconstructed image in different color canthen be observed, as shown in FIG. 5c.

FIG. 6 is a schematic representing an optical system for recording andreconstructing a Lippmann hologram. At the first stage, three firstholograms for R, G and B are produced as in the case of the rainbowhologram (see FIG. 6(a)). At the second stage, a second sensitivematerial 23 is located at the position of the first hologram where areal image is reproduced, and is then illuminated by reference lightfrom the opposite direction for treble exposure at varying recordingwavelengths R→G→B, thereby producing a second hologram, as shown in FIG.6(b). Upon reconstruction by the illumination of the second hologram bythe reference light and white light from the opposite direction, a colorimage is reproduced by the reflected and diffracted light, as shown inFIG. 6(c).

In addition, a surface relief type of hologram can be produced bycopying a mold having surface asperities in the form of interferencefringes.

To achieve displays in multicolor by the conventional method, pluralsets of interference fringes such as interference fringes 30 diffractingblue light, interference fringes 31 diffracting green light andinterference fringes 32 diffracting red light must be superposed on thesame hologram in the thickness direction in the case of a Lippmannhologram, as shown in FIG. 7(a). In the case of laser lightreconstructing and rainbow holograms, or "transmission holograms",plural sets of interference fringes such as interference fringes 30diffracting blue light, interference fringes 31 diffracting green lightand interference fringes 32 diffracting red light must be similarlyformed, as shown in FIG. 7(b). Not only does this give rise to technicaldifficulties, but also places some considerable limitation on thehologram recording material to be used, because the sensitive materialis required to be sensitive to plural colors. There is also adiffraction efficiency drop. Moreover, this is responsible for inferiorimage quality such as large noises, for instance, ghost images.

So far, heat-wave reflecting films have been put on the windows of carsor buildings so as to control a rise in the inside temperature. Atypical conventional heat-wave reflecting film has its surface designedto reflect heat waves in a preset wavelength range. To reflect heatwaves of wavelength longer than the preset wavelength, there is need ofadding to each site of the film surface a function capable of reflectingheat waves in a desired wavelength region. This has heretofore beenachieved by using a dielectric material and a metal or metal oxide thinfilm, etc.

However, conventional heat-wave reflecting films such as depositedfilms, except holograms, when used to reflect heat waves in a widewavelength range, result unavoidably in a lowering of the transmissionof visible light, because they are likely to reflect or absorb visiblelight. Diffraction gratings, although they may somehow be used asheat-wave reflecting films, will make it difficult to reflect heat wavesin a wide wavelength range due to their wavelength selectivity. Thisproblem may possibly be solved by superposing layers diffractingdifferent wavelength regions in the film thickness direction. However, aproblem with this superposition is that the light diffracted by onelayer is further diffracted by another layer, thus making it verydifficult to achieve any effective reflection of heat waves.

When interference fringes of a given pitch are recorded on aphotosensitive material, the diffraction efficiency shows a peak withrespect to the wavelength determined by that pitch. Hence, aphotosensitive material with interference fringes of a certain pitchrecorded thereon can be used as an optical filter, because itsreflectivity with respect to light of a given wavelength can beincreased. So far, this has been extended to a heat-wave reflecting filmhaving increased reflection properties with respect to the infraredregion, for instance. Also, methods of disturbing interference fringesby material treatments, thereby making the band width of the reflectionwavelength region wider, have been put forward.

However, it is extremely difficult to produce a wide band width of fitersuch as a solar reflector with the use of a diffracting grating withinterference fringes recorded thereon. In particular, a grave problemwith widening band width by conventional material treatments is that itis extremely difficult to regulate the reflection wavelength region to aspecific region.

One of well-known diffraction gratings is a volume hologram produced byrecording interference fringes on a film made up of photopolymer,dichromated gelatin, silver salt or the like by interference of light.However, this volume hologram has a narrow diffraction wavelength range;no volume hologram having a wide wavelength range is available as yet.Such a diffraction grating, when used in the form of a heat-wavereflecting film or the like, is required to have a diffractionwavelength range of at least a few 100 nm.

SUMMARY OF THE INVENTION

An object of the invention is to provide a hologram recording sheetwhich enables an image of good quality to be obtained and, at the sametime, hologram recording to be easily made at an advantageous cost.

Another object of the invention is to provide a hologram recording sheetthat can be used to make the pitch of interference fringes, as designed,either by simultaneous exposure or by sequential exposure.

A further object of the invention is to provide a hologram recordingsheet which enables an image of good quality to be displayed inmulticolor.

A still further object of the invention is to provide a hologramcombiner or display hologram which enables an image of good quality tobe displayed in multicolor.

A still further object of the invention is to provide a heat-wavereflecting film which can reflect heat wave wavelengths in a widewavelength region and can prevent any reduction in the transmission ofvisible light, and a method for producing it.

A still further object of the invention is to provide a holographicfilter which can easily achieve a broad band of filter characteristics,and a method for producing it.

A still further object of the invention is to provide a volume hologramtype of diffraction grating using a photopolymer, which has a broaderrange of diffraction wavelengths.

The invention is characterized by providing a hologram recording sheetin which at least two hologram materials sensitive to differentwavelength regions are formed on different regions on a film in a givenpattern, whereby, using laser light of a specific wavelength as ahologram recording light source, the regions diffracting light ofdifferent wavelengths can be recorded on the same plane in the form ofsets of interference fringes that are independent in the thicknessdirection of the photosensitive material.

Also, the invention is characterized by providing a hologram recordingsheet in which at least two hologram recording materials sensitive todifferent wavelength regions are laminated on a film through atransparent plastic spacer layer, whereby, using laser light of aspecific wavelength as a hologram recording light source, a plurality ofregions diffracting different wavelengths can be recorded on the sameplane either by simultaneous exposure or by sequential exposure.

Further, the invention is characterized by providing a hologramrecording sheet in which at least two hologram recording materialssensitive to different wavelength regions are formed on differentregions on a film in dotted or striped configuration of up to 200 μm orless in size, whereby, using laser light of a specific wavelength as ahologram recording light source, the zones diffracting light ofdifferent wavelengths can be formed on the same plane in the form ofsets of interference fringes that are independent in the thicknessdirection of the photosensitive materials.

Still further, the invention is characterized by providing a hologramrecording sheet in which at least two hologram recording materialssensitive to different wavelength regions are formed on differentregions on a film in dotted or striped configuration that is at leasttwice as large as the thickness of the photosensitive materials,whereby, using laser light of a specific wavelength as a hologramrecording light source, the regions diffracting light of differentwavelengths can be formed on the same plane in the form of sets ofinterference fringes that are independent in the thickness direction ofthe photosensitive materials.

Still further, the invention is characterized by providing a hologramcombiner or display hologram which is produced using a hologramrecording sheet in which at least two hologram recording materialssensitive to different wavelength regions are formed on different zoneson a film in dotted or striped configuration of up to 200 μm or less insize.

Still further, the invention is characterized by providing a heat-wavereflecting film in which diffraction gratings reflecting heat waves indifferent wavelength regions are formed on a surface in mosaicconfiguration, thereby enabling a wide wavelength range of heat waves tobe reflected.

Still further, the invention provides a holographic filter withinterference fringes recorded thereon, characterized in that sets ofinterference fringes of a plurality of different pitches are recorded.

Still further, the invention provides a method for producing aholographic filter by recording interference fringes, characterized inthat sets of interference fringes are recorded with a plurality of lightbeams different from each other in terms of the direction of incidenceon a photosensitive material, or in that sets of interference fringesare recorded with a plurality of light beams varying continuously interms of the direction of incidence on a photosensitive material.

Still further, the invention provides a diffraction grating made up of adistributed index type of interference fringes recorded on aphotopolymer, characterized in that the binder polymer used comprises amixture of a plurality of polymers different from each other in terms ofthe swelling properties with respect to a developer.

Still further, the invention provides a method for producing adiffraction grating made up of a distributed index type of interferencefringes recorded on a photopolymer, characterized by recording sets ofinterference fringes on a recording material obtained by dispersing areactive monomer and a photopolymerization initiator in a binder polymercomprising a mixture of a plurality of polymers different from eachother in terms of the swelling properties with respect to a developer,and then developing the recording material with a developer in which areactive monomer different from the reactive monomer in the recordingmaterial is dissolved.

Still further, the invention is characterized in that light is allowedto be diagonally incident on both sides of a photosensitive materialmade up of a plurality of hologram sensitive material layers withdifferent indices of refraction in the thickness direction, therebyproducing sets of interference fringes substantially parallel with eachlayer.

Still further, the invention is characterized in that light is allowedto be diagonally incident on one side of the photosensitive materialwhile a reflecting mirror is located on the other side thereof, therebyrecording sets of interference fringes.

Still further, the invention provides a method for producing adiffraction grating having a plurality of interference fringes ofdifferent pitches recorded thereon, characterized in that sets ofinterference fringes are recorded on the a photosensitive material bymultiple interference exposure using a plurality of light beams ofdifferent wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a conventional hologram sensitive material.

FIG. 2 is a schematic of a conventional color hologram.

FIG. 3 is a schematic of a hologram recording and reconstructing opticalsystem.

FIG. 4 is a schematic of an optical system for recording andreconstructing a laser light-reconstructing hologram.

FIG. 5 is a schematic of an optical system for recording andreconstructing a rainbow hologram.

FIG. 6 is a schematic of an optical system for recording andreconstructing a Lippmann hologram.

FIG. 7 is an imaginary schematic of interference fringes in aconventional hologram recording sheet for multicolor recording purposes.

FIG. 8 is a schematic of a hologram recording sheet according to theinvention.

FIG. 9 is a projection schematic of a head-up display combiner.

FIG. 10 is an imaginary schematic of interference fringes in a hologramrecording sheet for multicolor recording purposes according to theinvention.

FIG. 11 is a schematic of a hologram recording sheet provided with aprotecting layer.

FIG. 12 is a schematic of another example of the hologram recordingsheet according to the invention.

FIG. 13 is an imaginary schematic of interference fringes in a hologramrecording sheet for multicolor recording purposes.

FIG. 14 is a projection schematic of a head-up display combiner producedwith the hologram recording sheet shown in FIG. 12.

FIG. 15 is a schematic of a hologram recording sheet in which sensitivematerials recording different wavelengths are formed in dottedconfiguration.

FIG. 16 is a schematic of a hologram recording sheet in which sensitivematerials recording different wavelengths are formed in stripedconfiguration.

FIG. 17 is a schematic of a hologram recording sheet in which sensitivematerials recording different wavelengths are formed independently inthe thickness direction.

FIG. 18 is a schematic of a cell used for gravure printing.

FIG. 19 represents an optical path passing through a hologram.

FIG. 20 is a schematic of a hologram combiner or display hologram.

FIG. 21 is a schematic of Lippmann hologram's interference fringesproduced independently in the planar direction.

FIG. 22 is a schematic of laser light-reconstructing or rainbowhologram's interference fringes produced independently in the planardirection.

FIG. 23 is a schematic of Lippmann hologram's interference fringessuperposed independently in the thickness direction.

FIG. 24 is a schematic of laser light-reconstructing hologram'sinterference fringes produced independently in the thickness direction.

FIG. 25 is a schematic of an automotive head-up display.

FIG. 26 is a schematic of how to produce a heat-wave reflecting filmaccording to the invention.

FIG. 27 is a schematic of a light-blocking plate.

FIG. 28 is a schematic of a reflection wavelength region.

FIG. 29 is a schematic of how to produce a holographic filter accordingto the invention.

FIG. 30 is a schematic of the diffraction efficiency characteristics ofa filter according to the invention.

FIG. 31 is a schematic of how to produce a holographic filter having amirror in close contact with its back side.

FIG. 32 is a schematic of how to produce a holographic filter by meansof a plurality of point sources.

FIG. 33 is a schematic of how to produce a holographic filter by meansof a linear source.

FIG. 34 is a schematic of an example of mixing different swellingphotopolymers, thereby making the diffraction wavelength region wide.

FIG. 35 is a schematic of an example of laminating different swellingphotopolymers, thereby making the diffraction wavelength region wide.

FIG. 36 is a schematic of another example of the diffraction gratingwith a wider diffraction wavelength range.

FIG. 37 is a schematic of a slide coater arrangement.

FIG. 38 is a schematic of a slot coater arrangement.

FIG. 39 is a schematic of a curtain coater arrangement.

FIG. 40 is a schematic of still another example of the diffractiongrating with a wider diffraction wavelength range.

FIG. 41 is a schematic of the diffraction efficiency characteristics ofa diffraction grating produced according to the invention.

FIG. 42 is a schematic of another production method.

FIG. 43 is a schematic of a production method making use of a glassblock.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of the hologram recording sheet according to theinvention will be explained with reference to FIGS. 8 to 11, whereinreference numeral 41 stands for film, 42 a hologram sensitive material(sensitive to green), 43 a hologram sensitive material (sensitive tored), 44 a zone that diffracts green light, 45 a zone that diffracts redlight, 46 a display object to be projected in green (that is shown inthe form of a speed indicator), 47 a display object to be projected inred (that is shown in the form of an emergency warning light), 48interference fringes that diffract green light, 49 interference fringesthat diffract red light, and 50 a protecting layer.

In the hologram recording sheet according to this embodiment thehologram sensitive materials 42 and 43 are formed on the film 41 in adesired pattern, as shown in FIG. 8. The film 41 may be a base filmordinarily used as a dry film type of hologram sensitive material. Byway of example but not by way of limitation, use may be made of PET,TAC, Poly (vinyl chloride), polyethylene, PMMA. The hologram materials42 and 43 sensitive to different wavelength regions, which may be usedin the instant example, may be arbitrarily selected from sensitivematerials so far known to be used in hologram application, for instance,dichromated gelatin, silver salt, and photopolymers. However, preferenceis given to photopolymer sensitive materials different from each otheronly in terms of the absorption wavelength of the dye added as asensitizer. In FIG. 8, the hologram sensitive material 42 forms a zonesensitive to green light and the hologram sensitive material 43 forms azone sensitive to red light.

The printing of the hologram sensitive materials 42 and 43 sensitive todifferent wavelength regions on the film 41 may be achieved inwell-known manners, either by a continuous rolling-up procedure, e.g.,gravure, roll, blade or die coating or by an intermittent rolling-upprocedure, e.g., screen printing. In the case of such discontinuouspatterns as shown in FIGS. 8(a) and 9(a), particular preference is givento screen printing that assures alignment accuracy and highly uniformfilm thickness. In the case of such striped patterns as shown in FIGS.8(b) and 9(b), particular preference is given to relying on die coatingprinting due to its mass productivity.

By use of the thus obtained hologram recording sheet it is possible toselectively produce holograms at desired zones by desired laser light,because the sensitive materials sensitive to different wavelengthregions are used. For the recording light sources used to this end,various well-known lasers may be used, inclusive of helium neon laser,argon laser, krypton laser, helium cadmium laser and ruby laser.

In the instant example, the interference fringes, for instance, theinterference fringes 48 diffracting green light and the interferencefringes 49 diffracting red light are produced independently in thethickness direction, as can be seen from FIG. 10.

If the protecting layer 50 is provided on each hologram sensitivematerial 42 (43), it can then be prevented from being injured orotherwise damaged. It is noted that the protecting layer 44 may beprovided between each sensitive material and the film 41. In this case,if the hologram sensitive materials 42 and 43 are releasable from thefilm 41, the arrangement can then be used in the form of a transferfoil, etc.

EXAMPLE 1

For a solution sensitive to green light, 50 parts of polyvinylcarbazole,40 parts of tribromophenoxyethyl methacrylate, 1 part of cyanine dye(NK-1420 made by Nippon Kanko Shikiso Co., Ltd.), 5 parts of3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 5 parts of anonylphenyl alcohol/ethylene oxide adduct (Emulgen 903 made by Kao SoapCo., Ltd.) were dissolved in methyl ethyl ketone. For a solutionsensitive to red light, 50 parts of polyvinylcarbazole, 40 parts oftribromophenoxyethyl methacrylate, 1 part of squarylium dye (NK-3024made by Nippon Kanko Shikiso Co., Ltd.), 5 parts of3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 5 parts of anonylphenyl alcohol/ethylene oxide adduct (Emulgen 903 made by Kao SoapCo., Ltd.) were similarly dissolved in methyl ethyl ketone. Thesesolutions were printed on a 50 μm PET film (HP-7 made by Teijin Limited)by screen printing using a 150-mesh screen printing plate and then driedto form 10 cm·10 cm green-light sensitive zones and 2 cm·2 cm red-lightsensitive zone, each having a thickness of 20 μm, according to thedesired pattern shown in FIG. 8(a). The film was then fed 20 cm forsimilar printing. By repeating this, a hologram recording sheet wasobtained, on which the green and red hologram sensitive material layerswere arranged at an interval of 20 cm according to the desired pattern.This sheet was further laminated thereon with a PET film (Lumilar T60made by Toray Industries, Inc.) and then rolled up.

Using 514 nm argon laser light and 647 nm krypton laser light as lightsources, a head-up display combiner was produced on this hologramsensitive material. Consequently, a satisfactory speed-indicatingprojection chart was obtained, which included the speed indicator 46projected in green color in the green-light diffracting zone 44 and theemergency warning light 47 projected in red color in the red-lightdiffracting zone 45, as shown in FIG. 9(a).

EXAMPLE 2

Methyl ethyl ketone solutions of the same photosensitive solutions as inExample 1 were printed on a 50 μm PET film (HP-7 made by Teijin Limited)by die coating using a slot orifice having different discharge ports of25 cm and 5 cm, and then dried to form the green- and red-lightsensitive materials, each having a thickness of 20 μm, according to thestriped pattern shown in FIG. 8(b). This sheet was further laminatedthereon with a PET film (Lumilar T60 made by Toray Industries, Inc.) andthen rolled up.

Using 514 nm argon laser light and 647 nm krypton laser light as lightsources, a head-up display combiner was produced on this hologramsensitive material by the sequential exposure technique . Consequently,a satisfactory speed-indicating projection chart was obtained, whichincluded the speed indicator 46 projected in green color in thegreen-light diffracting zone 44 and the emergency warning light 47projected in red color in the red-light diffracting zone 45, as shown inFIG. 9(b).

According to the instant examples mentioned above, since the hologramsensitive materials that enable the necessary diffracted lightwavelengths to be recorded on the necessary zones are formed by patternprinting, a good-quality image can be obtained without producing anyunnecessary interference fringes. At the same time, since most ofhologram recording materials known so far in the art may be used assuch, hologram recording is easily achievable at advantageous cost.

Another embodiment of the hologram recording sheet according to theinvention will now be explained with reference to FIGS. 12 to 14, inwhich reference numeral 51 represents a film, 52 a hologram sensitivematerial (sensitive to green light), 53 a hologram sensitive material(sensitive to red light), 54 a spacer layer, 55 a protecting layer, 56interference fringes that diffract green light, 57 interference fringesthat diffract red light, 58 a display object (a speed indicator beingshown as an example) that is projected in green color, and 59 a displayobject (an emergency warning light being shown as an example) that isprojected in red color.

The hologram recording sheet according to the instant embodiment is madeup of such layers as shown in FIG. 12. The film 51 and hologramsensitive materials 52 and 53 are the same as explained in connectionwith FIG. 8. For the spacer layer 54, a film made up of a materialsimilar to that forming the base film, i.e., PET, TAC, Poly(vinylchloride), polyethylene, PMMA or the like, may be used. Alternatively,the spacer layer may be formed by coating a solution of these resins ina suitable solvent, followed by drying.

To obtain such layer configuration as shown in FIG. 12, for instance,the hologram sensitive material 52 (sensitive to green light) is firstprinted and formed on the base film 51 by known coating techniques suchas gravure coating, roll coating, blade coating or die coating. Then,the spacer layer 54 is laminated on the material 52. Finally, anotherhologram sensitive material 53 (sensitive to red light) is printed andformed on the spacer layer 54. In this case, the spacer layer 54 mayalso be formed by coating. In addition, techniques for preparing theabove-mentioned layer configuration at once by multi-layer extrusioncoating, multi-layer slide coating or the like may be applicable to theinvention. If an additional protecting layer 55 is provided on theabove-mentioned layer configuration, as shown in FIG. 12, it is thenpossible to prevent the hologram materials from being injured orotherwise damaged. It is noted that the protecting layer may be providedon the base film 51. In this case, if it is releasable from the basefilm 51, the layer configuration can then be used in the form of atransfer foil, for instance.

Then, a plurality of zones, in which the diffracted light differs inwavelength, are recorded on the hologram recording sheet of the abovelayer configuration in the form of two sets, independent as viewed inthe thickness direction of the sensitive materials, of interferencefringes. As shown, the interference fringes 56 that diffract green lightand the interference fringes 57 that diffract red light areindependently formed on both sides of the spacer layer 55. To producesuch a hologram, each of the hologram sensitive materials of thehologram recording sheet is illuminated by laser light having awavelength lying in the sensitive region peculiar to it.

In the case of a head-up display combiner shown in FIG. 14, asatisfactory speed-indicating projection chart including the displayobject 58 that is projected in green color (a speed indicator beingshown as an example) and the display object 59 that is projected in redcolor (an emergency warning light being shown as an example) is obtainedon the above hologram sensitive materials, using 514 nm argon laserlight and 647 nm krypton laser light.

According to the hologram recording sheet of the instant embodiment, thehologram sensitive materials sensitive to light of different wavelengthsare laminated on each other with the spacer layer located between them.Hence, even when a dry type of hologram sensitive materials are used asthe hologram sensitive materials, the migration of the monomer islimited to within each sensitive material layer, so that the pitch ofthe interference fringes can be established, as designed, irrespectiveof whether this is achieved by the simultaneous or sequential exposuretechnique.

EXAMPLE 3

The same green- and red-light sensitive solutions as in Example 1 weredissolved in methyl ethyl ketone at a solid content of 20 wt %. Theresulting solution for green light was coated on a 50 μm PET film (HP-7made by Teijin Limited) at a dry coverage of 25 μm by gravure coatingusing a gravure roll. Laminated on this layer was a 25 μm PET film (HP-7made by Teijin Limited) treated on both it sides, on which the sensitivesolution for red light was further coated at a dry coverage of 25 μm bygravure coating using a gravure roll. This sheet was laminated thereonwith an additional PET film (Lumilar T60 made by Toray Industries, Inc.)and then rolled up.

Using 514 nm argon laser light and 647 nm krypton laser light as lightsources, a head-up display combiner was produced on this hologramsensitive medium. Consequently, such a satisfactory speed-indicatingprojection chart as shown in FIG. 14 was obtained.

EXAMPLE 4

Methyl ethyl ketone solutions of the same photosensitive solutions as inExample 1 were provided. First, the solution for green light was coatedon a 50 μm PET film (HP-7 made by Teijin Limited) by die coating anddried to a 25 μm green-light sensitive material layer. Laminated on thislayer was then a 25 μm PET film (HP-7 made by Teijin) treated on bothits sides, on which the solution for red light was printed by diecoating and dried to obtain a 25 μm red-light sensitive material layer.This sheet was laminated thereon with an additional PET film (LumilarT60 made by Toray Industries, Inc.) and then rolled up.

Using 514 nm argon laser light and 647 nm krypton laser light as lightsources, a head-up display combiner was produced on this hologramsensitive medium. Consequently, such a satisfactory speed-indicatingprojection chart as shown in FIG. 14 was obtained.

According to these examples, since most of the hologram recordingmaterials known so far in the art may be used, hologram recording iseasily achievable at advantageous cost. In addition, since the hologramsensitive materials sensitive to light of different wavelengths arelaminated on each other with the spacer layer located between them,plural sets of interference fringes with varying pitches can beindependently produced, as designed, irrespective of whether this isachieved by either the simultaneous or sequential exposure techniques,so that a good-quality hologram can be recorded in multicolor.

A further embodiment of the hologram recording sheet according to theinvention will now be explained with reference to FIGS. 15 and 16.

As shown in FIGS. 15 and 16, hologram-recording sensitive materials 62,63 and 64 sensitive to blue, green and red light are formed on a film 61in dotted or striped configuration. FIGS. 15(a) and 16(a) are topschematics, FIGS. 15(b) and 16(b) cross-sectional schematics, and FIGS.15(c) and 16(c) partly enlarged schematics. The film 61 may be a basefilm ordinarily used for a dry film form of hologram sensitive material,and for this use may be made of PET, TAC, Poly(vinyl chloride),polyethylene, PMMA, etc. Also, the hologram-recording sensitivematerials 62, 63 and 64 sensitive to different wavelengths, used in theinvention, may be arbitrarily selected from sensitive materialsheretofore known in the art to be used in hologram applications, forinstance, bichromated gelatin, silver salt and photopolymers. Of these,preference is given to photopolymer sensitive materials differing fromeach other only in terms of the absorption wavelength of the dye addedas a sensitizer.

Such hologram sensitive materials 62, 63 and 64 sensitive to differentwavelengths may be printed and formed on the film 61 by gravure, screenor other known printing techniques.

When these materials are formed in dotted configuration by gravureprinting, as shown in FIG. 15, dot size and thickness may be arbitrarilydetermined depending on the cell geometry of the gravure. For instance,they may be printed in a given dotted pattern by using the gravure witha grating form of cells (FIG. 18(a)) or pyramidal cells (FIG. 18(b)).When reliance is on the screen printing technique, dot size andthickness may be determined by the count of the mesh and theconfiguration of the mask.

When these materials are formed in such striped configuration as shownin FIG. 16, hatched cells such as those shown in FIG. 18(c) are used, ifreliance is on the gravure printing technique. When reliance is on thescreen printing technique, what types of stripes are used may bedetermined by the count of the mesh and the configuration of the mask.

It is understood that, as shown in FIGS. 15 and 16, if protecting films68 are provided on the hologram sensitive materials 62, 63 and 64, it isthen possible to prevent them from being injured or otherwise damaged.In this case, the protecting films may be provided on the film 61. Ifthe hologram sensitive materials 62, 63 and 64 are releasable from thefilm 61, the layer configuration may then be used in the form oftransfer foils, for instance.

Various approaches are thought of for achieving the alignment of varyinghologram sensitive materials. However, the simplest approach is to makeuse of a marker separately provided on the film, as usually done forordinary printing. The key point is to reduce the leveling fortransferring the pattern neatly, but this may be achievable by takingcare of viscosity, solvent and cell geometry. Pattern width may bearbitrarily determined depending on the space between the cells or thenumber of the cells (the number of the cells per inch), while sensitivematerial thickness may be arbitrarily determined depending on celldepth. The larger the space between the dots or stripes, the easier theprocessing is. At a space exceeding 200 μm, however, they can be visibleto the naked eye in mosaic configuration. In other words, the resultingmulticolor display becomes impractical. Hence, the space must preferablybe as small as possible in view of image quality.

In the case of a Lippmann or reflection hologram, that separation mustbe of some magnitude in view of hologram performance. Since a generalphotosensitive material has a refractive index of nearly 1.5 and air hasa refractive index of 1, optical paths taken by the incident lightthrough the photosensitive material are shown at 71 and 72 in FIG. 19.

Now consider the optical path 72 taken by light that falls on thephotosensitive material at a position at an angle of nearly 90° withrespect to the normal line. The angle of incidence of light in thephotosensitive material is found by the diffraction equation to be:

    θ=sin.sup.-1 (1/1.5)=41.8°

so that, to reflect light within the size r in μm of the photosensitivematerial, ##EQU1## where a is the film thickness in μm. To diffractlight effectively, therefore, it is clear that the size of thephotosensitive material must be at least nearly twice as large as thethickness of the film. In the case of an optical path, like the opticalpath 71, taken by light that is incident at an angle smaller than thatincident on the optical path 72 in FIG. 19, the light is allowed to exitat a shorter radius.

Alternatively, a hologram recording film 61 on which layers sensitive tovarying wavelengths are coated in multi-layer configuration may be used,as shown in FIG. 17. It is here understood that FIGS. 17(a) and 17(b)are top and cross-sectional views, respectively. As shown, a protectinglayer 68 is formed on the uppermost layer. In this case, such problemsas explained with reference to FIG. 19 do not arise, because the layerconfiguration is flat in the areal direction.

With the hologram recording sheet produced by the above method in whichthe respective layers are different from each other in terms ofsensitivity to wavelength, it is possible to produce a hologram on adesired region by the selective illumination of laser light. Here,various lasers known so far in the art, for instance, helium laser,argon laser, krypton laser, helium cadmium laser and ruby laser may beused as recording light sources. It is understood that a relief hologrammay be produced by CGH (computer generated hologram).

A hologram combiner or display hologram, for which the hologramrecording sheet according to the instant embodiment is used, will now beexplained.

The hologram combiner or display hologram according to the invention ismade up of sets of interference fringes that are independently formed indotted configuration (see FIG. 20(a)) or striped configuration (see FIG.20(b)), or sets of interference fringes superposed in the thicknessdirection, as shown in FIG. 20(c). To produce the interference fringesin dotted or striped configuration, as shown in FIGS. 20(a) and 20(b),there is one possible method in which hologram recording is made using ahologram recording film 61 on which materials 62, 63 and 64 sensitive toblue, green and red light are printed in a mosaic pattern.Alternatively, the hologram recording film may be exposed to blue, greenand red light during hologram recording, while it is masked.

To produce sets of interference fringes independently in the thicknessdirection, as shown in FIG. 20(c), the hologram recording film 61 onwhich hologram sensitive materials sensitive to different wavelengthsare coated in multi-layer configuration, as shown in FIG. 17, issubjected to simultaneous or sequential exposure for hologram recording.Alternatively, these materials may be laminated together after recordingthe holograms thereon.

When a Lippmann hologram is used, regions 75, 76 and 77 for diffractingblue, green and red light, respectively, are formed independently in theplanar direction, as shown in FIG. 21. In the case of a transmissionhologram, too, a region for diffracting each of blue, green and redlight is formed independently in the planar direction, as shown in FIG.22.

These diffracting regions may be formed independently in the thicknessdirection. In the case of a Lippmann hologram, regions 75, 76 and 77 fordiffracting blue, green and red light, respectively, are formed, asillustrated in FIG. 23. In the case of a transmission hologram, too,these diffracting regions are formed independently in the thicknessdirection, as shown in FIG. 24.

It is understood that when the hologram recording sheet is used as anautomotive head-up display combiner, it is necessary that glasses 81 and82 be laminated together with a hologram combiner 80 sealed betweenthem, as shown in FIG. 25. In general, the hologram combiner is sealedtogether with a polyvinyl butyral resin 83, as shown in FIG. 25(a). Insome cases, the hologram combiner may come into close contact with thepolyvinyl butyral resin, resulting in some degradation of image quality.To avoid this, a barrier film 84 may be provided between the glasses, asshown in FIG. 25(b). To achieve an improvement in impact resistance, anadhesive layer 85 may be provided on each interface, as shown in FIG.25(c).

EXAMPLE 5

The same solutions sensitive to green and red light as in Example 1 wereprovided. Additionally, there was provided a solution sensitive to bluelight consisting of 50 parts by polyvinylcarbazole, 40 parts oftribromophenoxyethyl methacrylate, 1 part of cyanine dye (NK-723 made byNippon Kanko Shikiso Co., Ltd.), 5 parts of3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 5 parts of anonylphenyl alcohol/ethylene oxide adduct (Emulgen 903 made by Kao SoapCo., Ltd.) were dissolved in methyl ethyl ketone. These solutions,dissolved in methyl ethyl ketone at a solid content of 30%, weresequentially coated on a 50 μm PET film (HP-7 made by Teijin Limited) bythe gravure printing technique using gravure printing plates withvarious cell geometries, thereby producing the desired patterns shown inFIGS. 15 and 16. Each hologram recording sheet was laminated thereonwith a PET film (Lumilar T60 made by Toray Industries, Inc.) and thenrolled up. Set out in Table 1 are the pattern configurations, etc., ofthe obtained films.

                  TABLE 1                                                         ______________________________________                                        Configurations of Gravure Printing Plates                                       No.    Cell Geometry                                                                            Depth, μm                                                                             Number of Lines per inch                       ______________________________________                                          1 Grating 250  60                                                             2 Grating 250 175                                                             3 Grating 250 250                                                             4 Grating 250 400                                                             5 Pyramid 200  60                                                             6 Pyramid 200 175                                                             7 Hatched 150  60                                                             8 Hatched 150 175                                                             9 Hatched 150 150                                                           ______________________________________                                        Configurations of Sensitive Materials                                           No.    Geometry   Film Thickness, μm                                                                    Size, μm                                    ______________________________________                                          1 Dotted 20 250-400                                                           2 Dotted 20  80-150                                                           3 Dotted 20  60-100                                                           4 Dotted 20 30-60                                                             5 Dotted 15 200-300                                                           6 Dotted 15  60-120                                                           7 Striped 20 300-450                                                          8 Striped 20 100-200                                                          9 Striped 20  80-120                                                        ______________________________________                                    

EXAMPLE 6

The same photosensitive solutions as in Example 5, dissolved in methylethyl ketone, were screen-printed on a 50 μm (HP-7 made by TeijinLimited) in the striped pattern shown in FIG. 16. This sheet waslaminated thereon with an additional PET film (Lumilar T60 made by TorayIndustries, Inc.) and then rolled up. Set out in Table 2 are the patternconfigurations, etc. of the thus obtained films.

                  TABLE 2                                                         ______________________________________                                        Configurations of Screen Plates                                                                      Line Width Number of Lines                               No. Mask Geometry lines/inch lines/inch                                     ______________________________________                                        10    Hatched      60           150                                             11 Hatched 250   300                                                          12 Hatched 400  400                                                           13 Hatched 600  400                                                         ______________________________________                                        Configurations of Sensitive Media                                               No.     Geometry     Film Thickness, μm                                                                    Size, μm                                 ______________________________________                                          10 Striped 40 350-400                                                         11 Striped 40  80-120                                                         12 Striped 40 50-70                                                           13 Striped 40  5-20                                                         ______________________________________                                    

Hologram sensitive materials 1 to 13 were sequentially exposed to 458 nmargon laser light, 578 nm dye laser light and 647 nm krypton laserlight, thereby making head-up multicolor display combiners with thecharacteristics set out in Table 3. Through the optical system shown inFIG. 6 using blue (488 nm), green (578 nm) and red (647 nm) laser light,hologram recording was made with the hologram recording sheets on whichthe hologram materials sensitive to blue, green and red light wereformed with the configuration, film thickness and size shown in Table 1,thereby producing display holograms capable of diffracting 460 nm bluelight, 545 nm green light and 610 nm red light. These holograms arefound to have similar characteristics (see Table 3).

                  TABLE 3                                                         ______________________________________                                        Hologram Recording Sheets (Hologram Combiners)                                    No.     Configurations                                                                            Film Thickness, μm                                                                    Size, μm                                ______________________________________                                          14 Dotted 20 250-400                                                          15 Dotted 20  80-150                                                          16 Dotted 20  60-100                                                          17 Dotted 20 30-60                                                            18 Dotted 15 200-300                                                          19 Dotted 15  60-120                                                          20 Striped 20 300-450                                                         21 Striped 20 100-200                                                         22 Striped 20  80-120                                                         23 Striped 40 350-400                                                         24 Striped 40  80-120                                                         25 Striped 40 50-70                                                           26 Striped 40  5-20                                                         ______________________________________                                        Hologram Combiners (Image Quality of Display Holograms)                           No.    Image Appearance   Image Brightness                                ______________________________________                                          14 R, G and B separated Bright                                                15 Good Bright                                                                16 Good Bright                                                                17 Good Bright                                                                18 Good Bright                                                                19 Good Bright                                                                20 R, G and B separated Bright                                                21 Good Bright                                                                22 Good Bright                                                                23 R, G and B separated Bright                                                24 Good Bright                                                                25 --  Dark                                                                   26 -- Dark                                                                  ______________________________________                                    

EXAMPLE 7

Through the optical system shown in FIG. 5 using blue (488 nm), green(578 nm) and red (647 nm) laser light, holograms were recorded onhologram recording sheets on which the hologram recording materialssensitive to blue, green and red light were superposed with the filmthickness set out in Table 2, producing display holograms and hologramcombiners capable of diffracting 460 nm blue light, 545 nm green lightand 610 nm red light. Then, white light and CRT images were respectivelyprojected on these display holograms and hologram combiners. The imagequality obtained is set out in Table 4.

                  TABLE 4                                                         ______________________________________                                        Hologram Recording Sheets (Hologram Combiners)                                             Blue          Green Red                                            No. (μm) (μm) (μm)                                                 ______________________________________                                          27 10 10 10                                                                   28 20 20 20                                                                 ______________________________________                                        Image Quality of Display Holograms (Hologram Combiners)                             No.      Image Appearance                                                                           Image Brightness                                  ______________________________________                                          27 Good Bright                                                                28 Good Bright                                                              ______________________________________                                    

According to these examples, since the hologram sensitive materials thatenable the necessary diffraction wavelengths to be recorded on thenecessary regions are formed as by pattern printing in a dotted orstriped pattern with the size at least twice as large as the filmthickness of the sensitive materials or of up to 200 μm, good imagequality can be displayed in multicolor without producing unnecessaryinterference fringes. At the same time, most of hologram recordingsensitive materials known so far in the art may be used as such. Inaddition, hologram recording is easily achieved at advantageous cost.

It is noted that the hologram recording sheets according to the instantexamples can be used not only for multicolor head-up display combinersbut for ornamental and forgery-preventing purposes as well.

It is also noted that, by use of the sheet according to the instantexamples is it possible to produce a display hologram capable ofdiffracting at least two types of light, which achieves a good displayof sufficient brightness in multicolor without producing unnecessaryinterference fringes.

The heat-wave reflecting film according to the invention will now beexplained.

For the photosensitive layer used as the heat-wave reflecting filmaccording to the invention, ordinary photosensitive materials forholograms may be used. For instance, use may be made of a wet typematerial which is developed using polyvinyl carbazole as a binderpolymer to dissolve an unexposed region or a dry type material whichmakes use of the diffusion, etc., of a monomer during exposure, thusdispensing with any development step.

As shown in FIG. 26, light-blocking plates 92 are put on both sides of aphotosensitive material 91. As shown in FIG. 27, each light-blockingplate 92 is provided with a mosaic array of windows, each with one sideof a few cm². A glass block 93 or 94 having an end face at a desiredangle is put on the light-blocking plate 92 through an index matchingsolution 95. The index matching solution 95 used has an index ofrefraction close to that of the photosensitive material 91. The angle θof the end face of the glass block 93 or 94 is given by Eq. (1):

    sin θ.sub.1 =λ.sub.1 /2dn.sub.0               (1)

where λ₁ is the exposure wavelength, n₀ is the index of refraction ofthe photosensitive material and d is the spacing of the diffractiongrating to be recorded. This diffraction grating spacing d is then givenby

    d=λ.sub.2 /2n.sub.0                                 (2)

where λ₂ is the wavelength to be diffracted and n₀ is the index ofrefraction of the photosensitive material. Then, parallel laser lightbeams 96 and 97 are allowed to pass from the end faces of the glassblocks 93 and 94 through the photosensitive material 91 forinterference, followed by removal of the glass blocks and index matchingsolution (notice that this holds for the dry type). After this, asubstance having swelling action is injected under the windows in thelight-blocking plates 92. For this injection osmotic pressure may beused, and for the substance having swelling action a monomer with asmall structure or the like may be used.

After the interference fringes have been produced in this way,light-blocking plates with the positions of windows opposite to thoseshown in FIG. 27 are put on for similar treatments. In this case,however, the angles θ of the glass blocks are so varied that laser lightis allowed to be incident vertically on the end faces to produceinterference fringes with a different grating spacing. In the case ofthe wet type, solvent development may be done at the above step afterexposure. What type of developer is used may be determined depending onthe binder polymer used. For instance, if it is polyvinyl carbazole,acetone, methyl ethyl ketone, etc. can then be used, and if it is PMMAor its copolymer with styrene or the like, alcohol, ethylene glycolmonoethyl ether, etc., can then be used.

When 488 nm argon laser light is used in this manner, a heat wave near1,000 nm can be diffracted at a glass block angle θ of 61° and a heatwave near 1,400 nm at a glass block angle θ of 70°. It is here notedthat this is also achievable without recourse to the light-blockingplates. For instance, two types of interference fringes with a uniformgrating spacing may be used, if they are rearranged in a mosaic array,although the overall diffraction efficiency will drop. Alternatively,the area ratio may be changed depending on an energy reduction in thelonger wavelength side of the sunlight.

In this way, it is possible to produce a region which enables thediffraction efficiency to show a peak with respect to a plurality ofwavelength regions; in other words, it is possible to obtain a heat-wavereflecting film that can reflect heat waves in a wide wavelength range.

EXAMPLE 8

Light-blocking plates, index matching solutions and glass blocks (θ=60°)were laminated on both sides of a 20-μm thick film of Omnidex 352 (DuPont) in this order. Then, 488 nm argon laser light was split into twoparallel beams, which were then allowed to be incident vertically onboth the glass blocks for interference in the film. Interference fringescould be produced by exposure at 20 mJ/cm². The same procedure wasrepeated using light-blocking plates with the positions of the openingsopposite to those mentioned above and glass blocks (θ=70°). Further, theglass blocks, index matching solutions and light-blocking plates wereremoved, followed by the lamination of a color tuning film at 100° C.After this, the film sheet was post-baked at 130° C. for 1 hour. Theresulting film sheet was found to show diffraction peaks at around 1,000nm and 1,400 nm with a diffraction efficiency of 95%. The film sheet wasthen put on all the window glasses of a car and allowed to stand alone.As a result, it was found that the inside temperature was 4° C. lowerthan that of a car with no film sheet put on the windows.

EXAMPLE 9

A photosensitive solution consiting of 100 parts of astyrene-monoisobutyl maleate copolymer, 50 parts of trimethylolpropanetriacrylate, 8 parts of3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, 1 part of3,3'-carbonylbis(7-diethylaminocumarin) and 1,000 parts of dioxane wasapplied on a 3-mm thick, 20 cm·20 cm glass by means of an applicator,and then dried to obtain a 20 μm film, on which 10 wt. % of polyvinylalcohol was spin-coated to form an overcoat layer. As in Example 8,light-blocking plates, index matching solutions and glass blocks werelaminated on the film for exposure at 8 mJ/cm². Then, the procedure ofExample 8 was followed, using different light-blocking plates and glassblocks, followed by removal of the glass blocks, index matchingsolutions, light-blocking plates and overcoat layer. After this, thefilm sheet was dipped in ethylene glycol monoethyl ether for developmentand swelling. By measurement, this film sheet was found to showdiffraction gratings at 1,100 nm and 1,350 nm with a diffractionefficiency of 90%. Here, too, this film sheet, when estimated on a car,was found to have similar effects.

Thus, a heat-wave reflecting film having a wide range of diffractionwavelengths could be easily produced with high visible lighttransmission properties due to wavelength selectivity making use ofdiffraction. The heat-wave reflecting film obtained according to theinvention, when put on the windows of a car or building, makes itpossible to lower the inside temperature.

FIG. 29 is a schematic that represents how to produce the holographicoptical element according to the invention. Upon light beams L1, L2, L3and L4 with varying directions of incidence striking a photosensitivematerial 101 and a light beam L0 striking the opposite side thereof,interference fringes with a pitch varying dependent on the angle madebetween the light beams are recorded. As a result, characteristics withvarying peak wavelengths and a plurality of diffraction efficienciessuperposed on each other are obtained, as shown in FIG. 30. Moreover,there is an increase in the diffraction efficiency with respect to agiven wavelength region, so that the reflection characteristics withrespect to light in this wavelength region can be improved, thusenabling a band cut optical filter to be easily produced.

Alternatively, a plurality of light beams L1, L2, L3 and L4 with varyingdirections of incidence may fall on one side of a photosensitivematerial 101, while a mirror 102 is in close contact with the oppositeside thereof, thereby recording interference fringes, as shown in FIG.31.

FIG. 32 represents how to record interference fringes by a plurality ofdivergent point sources 103, 104 and 105 and, as illustrated,interference fringes with a pitch depending on a difference in theangles in the respective directions of incidence are recorded. Here,too, a mirror may be brought in close contact with the opposite side ofthe photosensitive material instead of using the light beam L0.

Still alternatively, a lens may be located between the light sources andthe photosensitive material.

Still alternatively, interference fringes may be recorded with one pointsource and a plurality of lenses.

FIG. 33 represents how to record interference fringes with acontinuously changing pitch and, as illustrated, a photosensitivematerial 101 is illuminated by light from a linear source 106. In thiscase, the light incident on the photosensitive material 101 changesdirection continuously, and so the pitch of the interference fringesrecorded changes continuously, so that a broad band of holographicfilter can be produced. Here, too, it is of course that a mirror may bebrought into close contact with the opposite side of the photosensitivematerial instead of using the light beam L0.

EXAMPLE 10

A mirror was put on the back side of a 40-μm thick Omnidex 352 (made byDu Pont) for multiple exposure by (514 nm) argon laser. To this end, therecording film was first illuminated at an intensity of 1 mW/cm² and anangle of 30° with respect to the normal line for 30 seconds. Then, itwas irradiated at an intensity of 1 mW/cm² and at an angle of 50° withrespect to the normal line for 30 seconds. After this, xylene was addeddropwise to the recording film, followed by the provision of a quartzglass prism thereon. The recording film was then irradiated at anintensity of 1 mw/cm² and an angle of 70° with respect to the normalline for 50 seconds, after which the film was irradiated all over thesurface at a 365 nm wavelength and 200 mj/cm² with the use of anultra-high-pressure mercury lamp (made by Fusion Co., Ltd.), followed bya two-hour heating at 120° C. The recording film was measured in termsof transmittance with the use of a double monochromatorspectrophotometer UV-365 made by Shimadzu Corporation. As a result, adiffraction grating was obtained, which was found to have peaks at threewavelengths 600 nm, 800 nm and 1,500 nm.

Thus, it is possible to easily record interference fringes with avarying pitch with the use of an optical system, and so it is possibleto easily produce a broad band of holographic optical filter.

The diffraction grating with a wider range of diffraction wavelengthswill now be explained.

When polymers different from each other in terms of the degree ofswelling with respect to a developer are simultaneously used as thebinder polymer of the photopolymer in the photosensitive layer, thedegree of swelling of the photopolymer due to the developer differs fromsite to site. To avoid this, a developer in which a monomer differentfrom that in the photopolymer is dissolved is used after recording bylight interference, whereby the unreacted monomer in the photosensitivelayer is substituted by the monomer in the developer in the process ofswelling. This causes the proportion of the monomer to be substitutedand hence the index of refraction to differ from site to site, and sothe optical distance between the interference fringes to differ fromsite to site, enabling the diffraction wavelength range to become wider.

For instance, when there are a high swelling portion 111 and a lowswelling portion 112, as shown in FIG. 34(a), all pitches d are the samein the state shown in FIG. 34(b), where the interference fringes arerecorded by exposure. Upon development, however, the pitches d' and d"of the high and low swelling portions 111 and 112 shown in FIG. 34(c)have the following relations:

    d'>d

    d"<d'

It is here noted that the pitches d, d' and d" of the interferencefringes vary depending on the degree of swelling, and, with this, thereconstruction wavelength varies. Hence,

    d=λ/2n·sin θ

where λ is the reconstruction wavelength, n is the index of refraction,and θ is the angle of incidence.

When interference fringes are recorded while high and low swellingportions 111 and 112 are laminated together, as shown in FIG. 35, allinterference fringe pitches d are the same in the recorded state. Upondevelopment, however, the pitch d' of the upper high swelling portionhas the relation d'>d, while the pitch d" of the lower low swellingportion has the relation d"<d'.

Therefore, by use of several binder polymers is it possible to make thediffraction wavelength region wider. For instance, this is useful forapplication to heat-wave reflecting films, etc.

More illustratively, the binder polymers used may includepolymethacrylate ester or its partial hydrolyzed product, polyvinylacetate or its hydrolyzed product, polystyrene, polyvinyl butyral,polychloroprene, polyvinyl chloride, chlorinated polyethylene,chlorinated polypropylene, poly-N-vinyl carbazole or its derivative,poly-N-vinyl pyrrolidone or its derivative, styrene-maleic anhydridecopolymer or its half ester, copolymers obtained by the polymerizationof a monomer selected from the group consisting of acrylic acid, acrylicester, methacrylic acid, methacrylic ester, acrylamide and acrylnitrile,and so on.

The monomers used may include light-polymerizable or crosslinkablemonomers having at least one ethylenically unsaturated double bond permolecule, their oligomers or prepolymers, or mixtures thereof. Examplesof the monomer and its copolymer are an unsaturated carboxylic acid orits salt, an ester of an unsaturated carboxylic acid with an aliphaticpolyhydric alcohol compound, an amide of an unsaturated carboxylic acidwith an aliphatic polyhydric amine compound, and so on.

Some binder polymers with varying swelling properties with respect tothe developer may be selected from the above binder polymers, and may becombined together for use. The selection of the polymers is efficientlyachieved by calculating the solubility thereof, since the ability ofthem to swell in the developer has a close relation to the solubility ofthem.

By calculation, the solubility parameter δ representing solubility isfound from the following equation developed by Fedors:

    δ=(ΣΔe.sub.i /ΣΔv.sub.i).sup.1/2

where Δe_(i) and ΣΔv_(i) are the evaporation energy and molar volume ofthe atom or atomic group, respectively. For instance, a δ value forn-butyl polymethacrylate is, by calculation, found to be 9.54.

More specifically and by way of example alone, a mixture of poly-N-vinylcarbazole (δ=12.4), a styrene-isobutyl maleate copolymer (δ=11.8) andpolymethyl methacrylate (δ=10.0) is used together with a mixed solventof acetone and ethyl cellosolve for development.

Similar effects are achievable with a mixture of some binder polymersthat are similar or analogous to each other in terms of structure buthave different weight-average molecular weights.

For instance, styrene-monoisobutyl maleate copolymers havingweight-average molecular weights of a few thousand, tens of thousandsand tens of ten thousands may combined together for use.

For the reactive monomers to be substituted, combinations of monomersdiffering largely in terms of the index of refraction may be selected.

For photopolymerization initiators and sensitizing dyes that areadditional components, use may be made of those ordinarily used in theart.

By exposing a photosensitive layer containing these components to laserinterference light and developing it, it is possible to obtain adiffraction grating having a wide range of diffraction wavelengths.

That is, the diffraction grating having a wide range of diffractionwavelengths, which is made up of a distributed index type ofinterference fringes recorded on a photopolymer according to theinvention, is characterized in that the binder polymer used comprises amixture of a plurality of polymers differing in terms of the degree ofswelling with respect to a developer.

Also, the method for producing a diffraction grating having a wide rangeof diffraction wavelengths, which is made up of a distributed index typeof interference fringes recorded on a photopolymer according to theinvention, is characterized in that the interference fringes arerecorded on a recording material obtained by dispersing a reactivemonomer and a photopolymerization initiator in a binder polymercomprising a mixture of a plurality of polymers differing in terms ofthe degree of swelling with respect to a developer, and the recordingmaterial is then developed with a developer in which a reactive monomerdifferent from the reactive monomer in the recording material isdissolved.

In this case, the polymers used may be different from each other interms of solubility, or may be similar to each other in terms ofstructure but have different molecular weights. Alternatively, they maybe different from each other in terms of structure as well as in termsof the rate of swelling with respect to the developer.

EXAMPLE 11

Dissolved in dioxane were a total of 50 parts of poly-N-vinyl carbazole,styrene-isobutyl maleate copolymer and styrene-acrylic acid copolymer asthe binder polymers, 45 parts of tribromophenoxyethyl acrylate as thereactive monomer, 3 parts of benzophenone as the photopolymerizationinitiator and 2 parts of 3,3'-carbonylbis(7-diethylaminocumarin) as thesensitizing dye, thereby preparing a 10 wt. % photosensitive solution.

This photosensitive solution was applied on a glass sheet with the useof an applicator, and then dried to form a 20-μm thick film.

Coated on this film was 10 wt. % of polyvinyl alcohol (PVA 205 made byKuraray Co., Ltd.) to form an oxygen barrier layer.

Then, this recording medium was exposed to 488 nm argon ion laser lightby the two-beam interference technique.

Subsequently, the oxygen barrier layer was released off with the use ofan adhesive tape.

The recording medium was then dipped for 30 seconds in a developerobtained by dissolving 50 wt. % of hydroxyethyl acrylate in a 1:1solution of acetone and ethyl cellosolve for the extraction of theunexposed tribromophenoxyethyl acrylate, and hydroxyethyl acrylate wasinjected into the swollen binder polymers.

After this, the recording material was irradiated with ultraviolet raysat 1 J/cm² with the use of an ultra-high-pressure mercury lamp, therebycuring the injected hydroxyethyl acrylate.

The thus obtained diffraction grating was found to diffract light of 470nm to about 650 nm.

It is noted that, to enlarge the diffraction wavelength range to a few100 nm, a plurality of diffraction gratings produced as mentioned above,each having a different diffraction wavelength region, may be laminatedtogether.

EXAMPLE 12

Provided for the binder polymers was a mixture of the following sixstyrene-maleic acid copolymers having different weight-average molecularweights (Mw):

Mw=1,700 (SMA17352)

Mw=1,900 (SMA2625)

Mw=2,300 (SMA3840)

Mw=2,500 (SMA1440)

Mw=105,000 (SCR1PSET550)

Mw=180,000 (SCR1PSET540)

The reactive monomer, photopolymerization initiator and sensitizing dyeused were the same as in Example 11. A photosensitive solutioncomprising these ingredients was likewise coated and dried, and anoxygen barrier layer was then likewise formed. The obtained recordingmedium was likewise exposed to light.

Following removal of the oxygen barrier layer, the recording medium wasdipped for 30 seconds in a developer obtained by dissolving 30 wt. % ofBiscoat 17F (a flourine-containing acrylate made by Osaka OrganicChemical Co., Ltd) in a 1:1 solution of isopropyl alcohol and ethylcellosolve for development.

After this, the recording medium was irradiated with ultraviolet rays at1 J/cm² from an ultra-high-pressure mercury lamp to cure the injectedBiscoat 17F.

The thus obtained diffraction grating was found to diffract light of 550nm to about 680 nm.

It is noted that, to enlarge the diffraction wavelength range to a few100 nm, a plurality of diffraction gratings as mentioned above, eachhaving a different diffraction wavelength region, may be laminatedtogether.

Comparative Example

Example 12 was followed with the exception that only SMA1440 withMw=2,500 was used as the binder polymer. However, the width of thediffraction wavelength region was as narrow as about 20 nm.

According to the invention, the binder polymer for the photopolymer ismade up of a plurality of polymers having different swelling propertieswith respect to the developer, as mentioned above. Thus, as therecording medium is developed with a developer in which a reactivemonomer different from the reactive monomer therein is dissolved, theunreacted reactive monomer in the recording material is substituted bythe reactive monomer in the developer, making the proportion of themonomer substituted and hence the index of refraction different fromsite to site. Hence, the optical distance between the interferencefringes differs from site to site, thereby enabling the diffractionwavelength range to be made wider.

With the diffraction wavelength range set at 800 to 2,000 nm, it ispossible to obtain a satisfactory heat-wave reflecting film thattransmits visible light but reflects heat waves. This heat-wavereflecting film, when put on the windows of a car or building, makes itpossible to reduce a rise in the inside temperature.

Another illustrative embodiment of the diffraction grating with awidened diffraction wavelength range will now be explained.

Referring to FIG. 36, hologram sensitive material layers 122 and 123 areformed on a glass substrate 121 to form a photosensitive sheet. Atriangular prism 125 is located on this sheet through a matchingsolution 124, while a mirror 126 is brought into close contact with thelower side of the glass substrate 121. The triangular prism 125 locatedat the uppermost position is to allow laser light to be efficientlyincident on the first photosensitive material layer 122 without causingany considerable refraction. The light incident on the first layer 122(n₁ =1.3) at an angle θ₁ (of about 57.4°) with respect to the normaldirection travels through the first layer, is refracted between thefirst layer 122 and the underlying second layer 123 (n₂ =1.7), andenters and travels through the second layer 123 at an angle θ₂ (of about40.1°) with respect to the normal direction. Then, the light passesthrough the glass substrate 121 and reaches the interface between theglass substrate 121 and the mirror 126, from which the light isreflected and, as shown, travels back through the second and firstlayers 123 and 122 at the same angle for the incident light. In thiscourse, the light traveling to the mirror and the light reflected by themirror interfere with each other in the respective layers 122 and 123 toproduce parallel interference fringes parallel in the respective layers.The thus produced interference fringes, when light falls vertically onthem, reflect light having a specific wavelength in dependence on thefringe spacing and the indices of refraction. In other words, thediffraction wavelength regions in the respective layers 122 and 123 aredifferent from each other.

To explain this briefly, now let n denote the index of refraction of therecording material and d represent the interference fringe spacing andassume that light, with the wavelength in vacuum represented by λ,travels through the recording material at the same angle θ with respectto the normal line and in the opposite direction. Then, the Braggcondition takes the form

    2nd sin (π/2-θ)=λ                          (3)

Hence, in the first layer 122,

    2n.sub.1 d.sub.1 sin (π/2-θ.sub.1)=λ       (4)

and in the second layer 123,

    2n.sub.2 d.sub.2 sin (π/2-θ.sub.2)=λ       (5)

Snell's law, on the other hand, can be written

    n.sub.1 sin θ.sub.1 =n.sub.2 sin θ.sub.2       (6)

Eqs. (4) to (6) can be applied to determining the spacings d₁ and d₂ ofthe interference fringes recorded in the respective layers 122 and 123.Based on the thus determined spacings, the reflection wavelengths λ₁ andλ₂ in the case of vertical incidence (θ=0) are then given by Eq. (3). Ingeneral, the wavelengths λ₁ and λ₂ differ from each other, if n₁ ≠n₂ andq₁ ≠0. By arbitrary selection of the wavelength λ of the recording laserlight, the refractive indices n₁ and n₂ of the material layers 122 and123 sensitive to that laser light, and the angle θ₁ of incidence of thelaser light, it is possible to select the range of the wavelength to bediffracted and hence widen the diffraction wavelength region. Further,by increasing the number of the photosensitive material layers is itpossible to make the diffraction wavelength range much wider.

It is noted that to coat the layers of hologram sensitive materialshaving different refractive indices on a plurality of base films,coaters such as those shown in FIGS. 37 to 39 may be used. FIG. 37represents a slide coater, FIG. 38 a slot coater, and FIG. 39 a curtaincoater, each enabling a plurality of layers to be coated at the sametime without giving rise to any increase in the number of the stepsinvolved. The coaters shown in FIGS. 37 to 39 will now be explainedbriefly. In each coater, the distal end of a nozzle 130 having aplurality of slits 131 to 133, through which the materials for therespective layers are extruded, is located in the illustratedconfiguration for the film 134 to be coated. Each of the slide andcurtain coaters shown in FIGS. 37 and 39 is designed to produce amulti-layer flow prior to coating and then coat it on the film 134, andso is suitable for coating a solution of low viscosity. In the formercase the distance between the nozzle and the film 134 is as small as 1mm or less, and in the latter case the nozzle may be spaced a fewcm-away from the film 134. The slot coater shown in FIG. 38 is designedto coat a plurality of layers on the film 134 in order from thelowermost layer, and so lends itself fit for coating a solution of highviscosity. Film thickness control may be achieved by the amounts of thesolutions extruded through the slits 131 to 133 in the cases shown inFIGS. 37 and 39, and by the distance d between the film 134 and thenozzle 130 in the case shown in FIG. 38.

As can be appreciated from what has been described, the method forproducing a diffraction grating with a wider diffraction wavelengthrange according to the invention is characterized in that aphotosensitive medium made up of a plurality of hologram sensitivematerial layers having different refractive indices in the thicknessdirection is provided, and light is allowed to strike diagonally bothsides of the photosensitive medium to record interference fringessubstantially parallel with the respective layers.

According to the invention, it is practical that the interferencefringes are recorded by allowing light to fall diagonally on one side ofthe photosensitive medium while a reflector mirror is located on theother side. More practically, the interference fringes can be recordedwith a higher efficiency, if light is incident on the face of the shortside of the triangular prism, while the face of the long side is broughtinto close contact with the other side of the photosensitive medium.

EXAMPLE 13

The triangular prism 125 shown in FIG. 36 is made up of glass BK7 (witha refractive index of 1.51). This triangular prism takes an isoscelestriangle form with a base angle of 46.5°. Parallel laser beams (647.1 nmkrypton laser light) are incident vertically on the face of one shortside of this isosceles triangle. The prism 125 and the photosensitivematerial are brought into close contact with each other with thematching solution 124 between them. The laser light passing through thetriangular prism 125 falls on the first layer 122 at an angle of 57.4°with respect to the normal direction. The light is then refracted andfalls on the second layer 123 at an angle of 40.1° with respect to thenormal direction. The light passing through the second layer 123 strikethe mirror 126 in close contact therewith through the matching solution,and is reflected thereby. The reflected light travels back through thesecond layer 123 at an angle of 40.1°. Further, the light is againrefracted and travels back through the first layer 122 at an angle of57.4° and then through the prism 125. At this time, the light travelingto the mirror 126 and the light reflected by the mirror 125 interferewith each other in the photosensitive materials 122 and 123 to produceinterference fringes parallel with the respective layers. The theninterference fringe spacing may be found by the following calculation.

The angle θ₁, at which the krypton laser light traveling through theprism 125 (with a refractive index of 1.51) at an angle of 46.5° isincident on the first layer (having a refractive index of 1.3), is givenby Eq. (4), i.e.,

    1.51×sin 46.5°=1.3×sin θ.sub.1

     θ.sub.1 =57.4°

The spaing d₁ of the interference fringes produced by the light incidentat this angle θ₁ =57.4° is given by Eq. (2), i.e.,

    2×1.3×d.sub.1 ×sin (90-57.4)=647.1

     d.sub.1 =462 nm

The actually reflected center wavelength λ₁ is found by substitutingn=1.3, d=462 and θ=0 for Eq. (3), i.e.,

    λ.sub.1 =1,201 nm

Similarly, the laser light is incident on the second layer 123 (having arefractive index of 1.7) at an angle of 40.1° with an interferencefringe spacing d₂ of 249 nm. At this time, the actually reflected centerwavelength λ₂ takes a value of 846 nm.

Practically, three layers of photopolymer materials having refractiveindices varying between 1.3 and 1.7 were coated on a glass substrate forimage-taking, while they were superposed on each other. A diffractiongrating produced from the recording material according to the aboveprocedure was found to cut off heat waves of 800 to 1,250 nm.

It is noted that the above example may be modified such that the laserlight is split as by a half-mirror into two beams, which are thenallowed to fall on both sides of the multi-layer photosensitive materialat the same angle of incidence for recording interference fringes.

According to the invention, a photosensitive medium made up of aplurality of hologram sensitive material layers having differentrefractive indices in the thickness direction is provided, and light isallowed to strike diagonally both sides of the photosensitive medium torecord interference fringes substantially parallel with the respectivelayers. Thus, the interference fringes recorded on the respective layersdiffer from each other in terms of the diffraction wavelength ofvertical incidence, so that a diffraction grating with an overall widediffraction wavelength range can be produced.

A further illustrative embodiment of the diffraction grating accordingto the invention will now be explained.

The diffraction grating according to this embodiment is made up ofvolume hologram interference fringes, and has plural sets ofinterference fringes of different pitches recorded thereon at the sametime. The respective sets of interference fringes are of so differentpitches that the diffraction and reflection wavelengths can differ. Thisenables the overall diffraction grating to have a wide diffractionwavelength range. It is desired, however, that the respective sets ofinterference fringes be produced parallel with the surface of thephotosensitive material.

Such a diffraction grating may be produced by multiple interferenceexposure using a plurality of light beams different in terms of thewavelength incident on the photosensitive material.

The diffraction grating according to the instant embodiment will now beexplained more specifically. When a diffraction grating having a wideneddiffraction wavelength region is produced by a limited number ofexposure wavelengths, some portion is reduced in terms of diffractionefficiency, although this depends on wavelength. Diffraction efficiencyη is found by the following equation (7). Alternatively, diffractionefficiency η is measured by a spectrophotometer, and the wavelengthinterval best suited for diffraction grating production by multipleexposure is determined in view of the relation of wavelength to the thusfound diffraction efficiency. For instance, multiple exposure may bedone at that wavelength interval, using a dye laser.

    η=ν.sup.2 /{ν.sup.2 +(ν.sup.2 -ξ.sup.2)/sin h.sup.2 (ν.sup.2 -ξ.sup.2).sup.1/2 }                        (7)

where

ν=iπΔnT/{λ_(c) (cos θ_(c) cos θ_(i))^(1/2) }

ξ=πT{n_(c) (cos θ_(c) -cos θ_(i))/λ_(c) -n_(o) (cos θ_(r) -cosθ_(o))/λ_(o) }

Here, Δn is the half-value of a refractive index change in theinterference fringes, T is the hologram thickness, θ_(c) is the angle ofillumination light during reconstruction, θ_(i) is the angle ofdiffraction light during reconstruction, θ_(r) is the angle of referencelight during recording, θ_(o) is the angle of object light duringrecording, λ_(c) is the reconstruction wavelength, and λ_(o) is therecording wavelength.

To shift the diffraction wavelength range to a wavelength longer thanthat in the taking wavelength range, it is required that image-taking bedone at a small angle of incidence. However, it is impossible for laserlight to strike a photosensitive material having a high refractive indexat an angle of incidence larger than the critical angle. In that case,laser light may be incident at any desired angle on the photosensitivematerial, while a glass block is brought into close contact with thephotosensitive material. It is thus possible to produce a diffractiongrating that can diffract light of longer wavelength in reconstructingit by vertical incidence.

How to produce the diffraction grating will now be explained moreillustratively with reference to FIGS. 40 to 43. Referring first to FIG.40 that represents how to produce the diffraction grating according tothe instant embodiment, a photosensitive material 141 is subjected tomultiple interference exposure using two light beams 142 and 143,thereby recording interference fringes of pitches depending on theexposure wavelength. As a result, diffraction efficiency properties witha plurality of superposed diffraction wavelength regions are achievable,as shown in FIG. 41. It is thus possible to easily obtain a diffractiongrating that shows a large diffraction efficiency in a given wavelengthrange, improves reflectivity with respect to light in this wavelengthregion, and has a wide diffraction wavelength range.

FIG. 42 represents another example of producing the diffraction grating.A plurality of light beams 142 having different wavelengths, asmentioned above, are allowed to be incident on a photosensitive material141 while a mirror 143 is brought into close contact with the back sidethereof, whereby the incident light and the light reflected from themirror 143 interfere with each other, making a multiple recording of theinterference fringes.

FIG. 43 represents an example of producing a diffraction grating with amirror 143 into close contact with the back side of a photosensitivematerial 141 and a glass block 144 in close contact with the surfaceside. More specifically, the glass block 144 is brought into closecontact with the photosensitive material 141 through a matching solution145, while the mirror 143 is in close contact with the back side. Byallowing a plurality of light beams 142 having different wavelengths tobe incident on the photosensitive material 141 is it possible to obtaina diffraction grating having diffraction characteristics in a longerwavelength region.

As can be understood from what has been explained above, the process ofproducing a diffraction grating having a wide diffraction wavelengthrange, on which a plurality of interference fringes having differentpitches are recorded according to the invention, is characterized inthat the interference fringes are recorded on a photosensitive materialby multiple interference exposure using a plurality of light beamshaving different wavelengths.

Using a dye laser as the light source, the recording material issubjected to multiple exposure using a plurality of light beams with thewavelength interval of up to 50 nm, whereby high diffraction efficiencyis similarly achievable at any wavelength in the diffraction wavelengthregion.

Such a diffraction grating can be easily produced by reflecting incidentlight from the back side of a photosensitive material while a reflectingmirror is located on the side of the photosensitive material opposite tothe incident side thereof, thereby recording interference fringesproduced by the interference of the incident light and the reflectedlight. In this case, if a transparent block is brought into closecontact with the incident side of the photosensitive material such thatthe incident light can strike the photosensitive material through thetransparent block, it is possible to obtain a diffraction grating havingdiffraction characteristics in a longer wavelength region.

EXAMPLE 14

A 20-μm thick photosensitive film of Omnidex 352 (made by De Pont) wasused. Laser light from a dye laser used as a light source was split intoparallel two light beams, which were then allowed to fall vertically onthe photosensitive film for quadruple exposure at an increment of 30 nmfrom 450 nm to 540 nm. Interference fringes were produced at theexposures of 3, 4, 5 and 6 mJ/cm². Following this, the photosensitivefilm was post-baked at 120° C. for 1 hour. As a result, a diffractiongrating having a diffraction wavelength region of 100 nm or more couldbe produced.

EXAMPLE 15

A 20-μm thick photosensitive film of Omnidex 352 (made by De Pont) wasused. A glass block and mirror for allowing light to be incident thereonat an angle of 60° are brought into close contact with the film, usingxylene as a matching solution. Using a dye laser as a light source, thefilm was subjected to quadruple exposure at an incremental of 25 nm from450 nm to 525 nm. Interference fringes were produced at the exposures of3, 4, 5 and 6 mJ/cm². Following this, the photosensitive film waspost-baked at 120° C. for 1 hour. As a result, a diffraction gratinghaving a 900 nm to 1,100 nm diffraction wavelength region could beproduced.

Thus, since the interference fringes are recorded on the photosensitivematerial by multiple interference exposure using a plurality of lightbeams having different wavelengths, it is possible to produce adiffraction grating having a wide diffraction wavelength region. Withthe diffraction wavelength range set at 900 to 1,100 nm, it is possibleto obtain a satisfactory heat-wave reflecting film that transmitsvisible light but reflects light in the infrared region.

What we claim is:
 1. A holographic optical element that diffracts atleast two types of light, characterized in that volume phase hologramdiffraction gratings, each diffracting light of a different wavelengthcorresponding to one of said at least two types of light, aredistributed in the planar direction in different areas on the samesurface of said holographic optical element in the form of sets ofinterference fringes that are independent in the thickness direction ofthe holographic optical element, said different areas having haddiffering spectral sensitivity.
 2. A holographic optical elementaccording to claim 1, characterized in that the independent sets ofinterference fringes are formed in dotted or striped configuration, thesize of which is 200 μm or less.
 3. A holographic optical elementaccording to claim 1, characterized in that the independent sets ofinterference fringes are formed in dotted or striped configuration, thesize of which is at least twice as large as the thickness of thesensitive material layers.
 4. A holographic optical element according toany one of claims 1 to 3, which is a hologram combiner or displayhologram.
 5. A holographic optical element according to claim 1 whereinthe volume phase hologram is a Lippman hologram.
 6. A holographicoptical element according to claim 1 wherein the volume phase hologramis a transmission type hologram.
 7. A holographic optical element,characterized in that it has a high transmittance for visible light andis formed such that a plurality of sites reflecting different regionsare distributed in the planar direction in different areas on the samesurface of said holographic optical element each of, said sites beingsets of interference fringes forming a volume phase hologram that areindependent in the thickness direction of the holographic opticalelement, such that the transmittance of heat waves having a wavelengthof 800 nm or larger is lowered, said different areas having haddiffering spectral sensitivity.
 8. A holographic optical elementaccording to claim 7, characterized in that the plurality of sitesreflecting different wavelength regions are arranged in mosaicconfiguration.
 9. A holographic optical element according to claim 7 or8, characterized in that diffraction gratings with different gratingspacings are formed on the sites reflecting different wavelengthregions.
 10. A holographic optical element with interference fringesrecorded thereon, characterized in that volume phase hologramdiffraction gratings in the form of sets of interference fringes of aplurality of different pitches are distributed in the planar directionin different areas on the same surface of said holographic opticalelement, and said sets of interference fringes are independent in thethickness direction of the holographic optical element, said differentareas having had differing spectral sensitivity.